There are many licenses that have fewer requirements than a GPL or copyleft license. People can take that code and make their own versions that are proprietary/

There are many licenses that have fewer requirements than a GPL or copyleft license. People can take that code and make their own versions that are proprietary/closed source for example. If the license is "compatible" then it allows anything the GPL or copyleft requires and is similar to a company taking the code and making it proprietary as far as the original license is concerned.